A method of printing images on halves of both sides of sheets is described in U.S. Pat. No. 4,334,765 and its corresponding European Patent Application No. 41,602. To make a book of prints from a group of numbered originals by this method, one half of the first side of a number of sheets are printed with half the odd-numbered originals. The other half of the odd-numbered originals are printed on one half of the second side of the number of sheets. Half the even-numbered originals are then printed on that half of the first side of the number of sheets which are still unoccupied by an image and the other half of the even-numbered originals are printed on that half of the second side of the sheets which are still unoccupied by any image.
The disadvantage of this method is that the sheets which together are required to form the book must be deposited on a stack after an image has been printed on each sheet and the sheets thereafter have to be individually withdrawn from the stack to print the next image thereon. This readily leads to errors in printing images in the correct sequence on the sheets, for example, because sheets that have been joined together continue to stick to one another and pass through the apparatus together. An error of this kind often cannot be corrected and usually causes the entire book production to be unusable.
The problems involved with refeeding copy sheets through a printing station for double-sided copying are different from the problems involved with refeeding original in single-sided copying. U.S. Pat. No. 4,508,447, Japanese Abstract No. 57-176029 and Japanese Abstract No. 60-29736 all relate to handling originals in single sided copying. They do not address the problems of refeeding copy sheets in double-sided copying.
U.S. Pat. No. 4,188,881 describes a method of printing images on halves of only one side of copy sheets and requires that the copy sheets must be cut into two packs which are assembled to form a set of single-sided copies in page order. U.S. Pat. No. 4,194,832 requires zigzag folding, cutting and trimming to form the finished book. Neither of these references describes copying on both halves of both sides of the sheet so that the resulting book can be formed by folding in half.
Therefore, it would be desirable to provide a method and device for printing images on halves of both sides of sheets, such that when the printed sheets together are folded double, they form a book without the disadvantages referred to above.